Re: [PATCH 3/3] Add timeout feature

On Wed, Jul 09, 2008 at 07:09:00AM -0400, Theodore Tso wrote:

quoted

Bloody hell! Doesn't *anyone* understand that a frozen filesystem is
*clean*? That the process of freezing it ensures all dirty data and
metadata is written out before the freeze completes? And that once
frozen, it can't be dirtied until unfrozen?

What do you mean by "it can't be diritied until unfrozen".  What
happens if I have a kernel compilation happening on a filesystem which
I am trying to freeze?   Does

(a) the freeze fail (because the checks equivalent to what happens
when you remount a filesystem read-only happen)?

(b) The process gets a kill -9 when it tries to write a file on the
frozen filesystem?

(c) The process gets a kill -STOP when it tries to write
to a file on the frozen filesystem?  

(d) The process won't fail, but just continue to run, filling the page
cache with dirty pages that can't be written out because the
filesystem is frozen?

(e) none of the above.  The kernel compilation will appear to pause
until the filesystem is unfrozen. No other visible effect should
occur. It will get blocked in a write or filesystem transaction
because the fs is frozen.

Look at vfs_check_frozen() - any call to that will block if the
filesystem is frozen or being frozen. The generic hook is in
__generic_file_aio_write_nolock() and various other filesystems have
calls in their specific write paths (fuse, ntfs, ocfs2, xfs, xip) to
do this.

For all other modifications, filesystem specific methods of
blocking transactions are used. XFS uses vfs_check_frozen() in
xfs_trans_alloc(), ext3 (and probably ocfs2) do it via
their ->write_super_lockfs method calling journal_lock_updates(),
ext4 via jbd2_lock_updates() and so on....

When the filesystem is unfrozen the journal is unlocked and
anything sleeping on the vfs_check_frozen() waitqueue is
woken.....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com